1. Field of the Invention
The present invention relates to a surround decoder capable of generating left and right stereo output signals, a center output signal and a surround output signal, and more specifically, to a surround decoder in which generation of unnecessary surround output signals is prevented in receiving a monaural signal.
2. DESCRIPTION OF THE BACKGROUND
An article regarding to Dolby Pro Logic Surround System proposed by Dolby Laboratories Licensing Corp. of the United States appears on pp. 88-89 in "NIKKEI Electronics" issued on June 27, 1988. As shown in FIG. 2, the system is comprised of left and right input terminals 1 and 2 to which left and right stereo signals L.sub.T and R.sub.T applied, a first adder circuit 3 for adding the left and right stereo signals L.sub.T and R.sub.T to generate a sum signal C' (=L.sub.T +R.sub.T), a subtractor circuit 4 for subtracting the right stereo signal R.sub.T from the left stereo signal L.sub.T to generate a difference signal S' (=L.sub.T -R.sub.T), first and second detection circuits 5 and 6 for respectively detecting levels of the left and right stereo signals L.sub.T and R.sub.T, third and fourth detection circuits 7 and 8 for respectively detecting levels of the sum and difference signals C' and S', a first level ratio detection circuit 9 for detecting level ratio of output signals of the first and second detection circuit 5 and 6, a second level ratio detection circuit 10 for detecting level ratio of output signals of the third and fourth detection circuits 7 and 8, a VCA (Voltage Controlled Amplifier) 11 including a plurality of gain controlled amplifier circuits (not shown) each controlling the level of the left or right stereo signal L.sub.T or R.sub.T in response to any one of output signals of the first and second level ratio detection circuits 9 and 10, a second adder circuit 12 for selectively adding the left and right stereo signals and output signals of the VCA 11 to generate left and right stereo output signals L and R, a center output signal C and a surround output signal S, a center mode control circuit 13 for switching between the left and right stereo output signals L and R and the center output signal C in response to a mode, and a passive decoder 14 for performing signal processing such as delay and noise reduction to the surround output signal S. Therefore, a structure such as shown in FIG. 2 enables a signal processing of enhancement of direction to clarify surround localization of sound, so that acoustics having presence can be provided to listeners. Particularly, the system is effective when applied to an audio signal processing of a large-sized television, so that it can produce the same effect on audience as that is obtained in seeing a picture at a theater.
However, if a monaural signal is received by means of the surround-decoder employed in the system, unnecessary signals are generated from a surround output terminal from which basically no output signal is generated. More specifically, noise generated in a transmission system or crosstalk component of other channel signals are generated at the surround output terminal as an output signal, thereby causing listeners to feel incompatible or uncomfortable.